In recent years have been developed technologies around the plastics industry, specifically in the polymer films directed to the packaging, which may be packaging for medical devices, foods, among others. These packages need to have specific properties that allow them to be subjected to processes such as thermoforming and high vacuum to ensure the quality of the product or products that contain.
Specifically, the food industry uses flexible thermoforming films for high vacuum packaging of products by presentation tray type with lid. The main problem of this process is the thinning of the corners and folds of the tray by not giving the films the ability to distribute its thickness along the perimeter of the mold. That is, the walls and floor are thicker than the corners. The corners and folds, by being thinner, reduce the gas barrier capacity affecting the shelf life of the packaged product.
To achieve a thickness of not less than 1 mil of an inch in the folds and the corners, films with thicknesses in proportion to 1 mil of an inch for each 1 cm of depth of the tray are used. This factor is derived to ensure a minimum thickness of 1 mils in the folds and the corners, it is to be considered that much of the thickness will remain in the walls and the floors. So that if a tray is 7 cm in depth, it is recommended to use a film of 7 mil of inch.
Some of the technologies developed in the area of packaging:
Canadian patent number CA2291207 details laminated films formed on cold, for example, for the manufacture or the blister type packaging, or the bases for the shapes of the containers or profiled parts for medical instruments packing and applicators. The film contains a barrier layer, which is impermeable to gases and water vapor, and a layer of plastic on both sides of the barrier layer. Laminated film has a layer structure in the following order: a.) a plastic layer in the form of a film of the following type, namely, polyvinyl chloride, polyester, polypropylene, polyamide, cycloolefin copolymer or a molten film of polyvinyl chloride or polypropylene, b) a metal sheet, such as an aluminum plate, and c) a polyamide oriented film that has a thickness of 25 to 32 um; the free sides from the first and/or second plastic film may have a sealing layer or films of the first and/or the second of plastic can be sealable. The laminated films can be processed, for example, by cold type stretching or forming mechanic tools. During the preparation of laminated film, the stretching is safely maintained between the matrix and the support, while the shaping tools keep the laminated in the desired form; it is mentioned the development of a plastic film obtained by laminated, and not by blowing as the invention to be protected, however it is quoted within this chapter since the inventors make emphasis in that the film or sheet to be molded to its final form through shaping tools in the absence of heat, maintains its thickness avoiding the gases or steam filtration, however it is done by cold means.
The Mexican Patent Application No. MX2009001376 relates to a film of multiple substrates, heat shrinkable, thermoforming, and sealable having a symmetrical construction around an internal substrate of two substrates and that has a heat shrink capacity in the machine direction and in the transverse direction in each case of at least 20% to 93° C. The heat shrinking capacity is not substantially affected by the thermoforming, and also relates to methods by which the film with multiple substrates of the invention can be processed advantageously for a heat shrinking packing, more particularly using packing machines specially packaged. This reference provides a multiple substrates film, heat shrink, thermoforming, and sellable, however the present invention to facilitate their thermoforming, includes a previous cross-linking process by electron irradiation. In the same way it's necessarily a symmetric film (the polymer of the inner and outer layers must be equal, a polyolefin). Do not use the three bubbles process, so the novelty of the present invention is not affected.
Entering a little more in detail on the processes that are followed in the thermoforming, the routine process that high vacuum thermoforming and packing machine uses is:
a) the equipment is fed with a high thickness film (Bottom Web).
b) This film is heated by electric resistances by determined time and temperatures by the film maker.
c) By means of vacuum, the film is sucked up to form a cold mold(s) which is underneath of it.
d) when contacting the sides of the mold, the film solidifies and gives shape to a tray.
e) This tray advances and food packed is deposited on it (sausage, cheese, meat, etc.).
f) The tray continues advancing and over the tray is applied a film (Top Web or Lidding Film), normally printed, and both enter into a vacuum chamber and sealing.
g) Packages already packed by high vacuum are cut to be marketed.
Commonly, the films for food thermoforming and packing are manufactured in a bubble co-extrusion equipment and in flat sheet co-extrusion equipment, these films can be mono-layer or multi-layer (2 or more layers). Additionally, in some cases to these co-extrusions are laminated with other mono or multi-layer plastic film, to give the required thickness, which in the industry is approximately one mil of inch for each inch of depth (1 mil/1 cm) of the tray to be formed, by which it should be noted that the most important one of these films is their property of being evenly formed. It is often the case that when being formed, some films become thin over the corners or folds on the tray, since the entire thickness of the film remains on the sides and base not in the corners or folds, and when the thickness is less than one mil, there is a risk that the barrier of the film is lost to such a level that affect the shelf life of the food.